It is often necessary to branch an optical fiber into more than two output fibers. Typical applications requiring such branching are found in fiber optic communication distribution systems. It is customary, if not necessary, to branch fibers in binary steps, e.g., 1 fiber branches into 2, 4, 8, or 16 fibers. In such a coupler, or splitter, the percentage of input light distributed to each of the output branches should be constant over a range of environmental extremes. This percentage should be stable over relatively wide thermal regimes, e.g., -40 to 80 degrees centigrade, should be independent of the input optical polarization state, and should be substantially constant over a range of optical wavelengths envisioned for the optical system. The intrinsic, or built-in, uniformity of the percentage of input light delivered to each output depends upon the repeatability of the manufacturing process.
Optical fiber couplers can be made by fusing two or more optical fibers during a tensioned elongation according to a process that has become familiar to those skilled in the art known as fused tapering. Key to the success of this operation is control of the placement of fibers prior to fusion. Fibers must be made to contact each other in the region to be heated and fused. Imperfections in or dirt on the surfaces of fibers, minor imbalances in tension forces applied to the fibers prior to application of fusion heat, and three dimensional non-uniformity of the heat applied are among factors which account for a low success rate of fused tapered couplers in manufacturing setups.
One prior approach to establishing and maintaining reliable contact among fibers prior to and during fusion employs a surrounding tube of optical material. Fibers in the set to be fused are placed inside a tube of lower refractive index than the cladding material of the fibers. The entire assembly is then heated and drawn in the usual manor. This approach can yield very good results in terms of uniformity and excess loss.
Couplers made using a surrounding optical media can be made to have a degree of wavelength tolerance over the 1200 nm to 1600 nm optical wavelength band. According to the methods of prior art, such 1.times.7 couplers are drawn until substantially equal power is output from all seven fibers at two predetermined wavelengths, e.g., 1300 and 1500 nm. When this is done the six surrounding fibers demonstrate a variation of optical coupling of the order of 3.0 percent over wavelength range of 1200 to 1600 nm. The central fiber, however, demonstrates more than 12% variation of power coupling over that same range of wavelengths. The tubed 1.times.7 coupler is therefore most suitable as a 1.times.6 coupler when wavelength stability is required.
The surrounding tube approach also has the disadvantage that the surrounding tube must be made to fit rather snugly around the fibers. Fibers used for coupler fabrication typically have an outer polymer layer known as the jacket. A tube sized to fit snugly around, e.g., seven fibers should have an inside diameter slightly greater than 3 times the diameter of the unjacketed fiber. Geometrical considerations dictate that such a tube cannot be fitted around seven fibers unless the jacketing material is removed from all of the fibers before the fibers are located in the tube. Since the fused coupler is formed in a short central region of longer fibers after the jacket is removed from all fibers in that region, it is inherent that the tension control mechanisms of most coupler manufacturing stations require that continuous fibers must be used. Tension is applied by holding the fibers on both sides of the central region in clamps attached to translation stages, often called drawing stages. Feeding long fibers into the tube is time consuming and has a high risk of contaminant transfer. This requires clean room environments for reliable coupler manufacturing.
Moreover, while prior art shows it possible to obtain reasonably good quality couplers by employing a surrounding optical medium to position the fibers prior to and during fusion, the approach does not lead to the desired 1.times.8 configuration and is difficult, if not impossible, to adapt to economical manufacturing.
Other approaches to fabricating 1.times.8 couplers generally entail tree structures wherein 1.times.2 couplers are spliced together. Typically a first 1.times.2 coupler splits the light input on one fiber into two outputs. These two outputs pass to the input of two more 1.times.2 couplers giving 4 outputs. These 4 outputs are then split again using four more 1.times.2 couplers yielding a total of eight outputs. Such a tree requires seven 1.times.2 couplers acquire a 1.times.8 split. The performance parameters of each of the seven component coupler becomes critical to the final performance of the tree assembly. Splicing losses must be carefully managed. The overall assembly is bulky and fragile because of the vulnerability of the interconnecting fibers between couplers in the tree. Moreover, a seven coupler assembly comprising a 1.times.8 tree has seven fused coupling regions and as many as 6 splice points. The failure of any one of these requires replacement of the entire tree assembly.
Considering the present state of the art in coupler fabrication technology, there is no economical means to make a robust, broadband 1.times.7 or 1.times.8 coupler. The invention addresses that need.